Semiconductor integrated circuits are typically made from thin wafers cut from silicon ingots known as boules. Cutting a wafer from a boule generally leaves the surfaces of the wafer in a rough condition, so wafers are polished on wafer polishing machines prior to starting semiconductor processing operations. The difficulty in achieving desired values of flatness and surface roughness increases as the diameter of the wafer to be processed increases and as the size of semiconductor structures (also known as “feature size”) to be fabricated on the wafer decreases. Wafer diameters have steadily been increasing and feature sizes decreasing at the same time that manufacturers have been pressured by market forces to increase manufacturing throughput and reduce manufacturing costs.
In the past, the relatively small size of wafers permitted a single wafer polishing machine having one or more head assemblies, each head assembly adapted to hold a plurality of wafers, to flatten and smooth many wafers simultaneously. Polishing machines use an abrasive, corrosive slurry to mechanically and chemically remove microscopic projections from the surface of a wafer. Machines for polishing bare wafers and machines for polishing by a chemical and mechanical process are known in the art. A wafer polishing machine has a horizontal rotating platen in a table base with a polishing pad attached to the top of the platen. A lid attached to the table base has at least one head assembly that is rotated during polishing. A wafer carrier attached to a head assembly holds one or more wafers to be polished. Pumps deliver slurry at a selected rate to the polishing pad and motors rotate the platen and head assemblies. Parts of the head assembly for carrying wafers have vertical travel relative to the surface of the polishing pad and may be raised or lowered to contact the polishing pad and to apply a selected amount of pressure to the surface of the wafers to be polished.
One or more wafers to be polished are attached to a wafer carrier and a wafer carrier is attached to each head assembly. Next, slurry is deposited on the polishing pad. The lid with wafers attached to the carriers on the head assemblies is lowered to enclose a polishing envelope and bring wafers closer to the polishing pad, and slurry is deposited on the polishing pad. Separate drive motors for the platen and head assemblies enables independent control of speed and direction of rotation. Polishing continues until the wafers achieve a desired value of wafer material removal, a desired value of surface quality, or a combination of both.
A quality and a rate of wafer polishing depend in part on a magnitude and direction of motion of the wafers relative to the polishing pad. The relative motion between the wafers and the polishing pad includes a component of rotational motion from the platen combined with a component of rotational motion of the head assembly to which the wafer is attached. In the case of a head assembly having a carrier holding a plurality of wafers, rotation of the head assembly results in wafer rotation relative to the platen and orbital motion of each wafer to and from the center axis of the platen. As technology progresses, the diameter of processed wafers also increases and the number of wafers that fit onto a carrier is correspondingly reduced. Furthermore, as wafer diameter increases, an edge of the wafer moves closer to the rotational center of a head assembly. The contribution to the rate of polishing by the rotation of the head assembly decreases for those parts of the wafer that are closest to the center of rotation of the head assembly. Some wafers are large enough that only one wafer may be placed in the central area of a carrier on a head assembly, in which case the component of radial, orbital motion from rotation of the head assembly is effectively lost in the central area of the wafer, and the quality of polishing is significantly degraded.
To achieve high quality polishing for large wafers, for example wafers having a diameter of 300 millimeters (12 inches), some polishing machines have only one head assembly above the platen. However, having only one head assembly per platen significantly reduces a rate of production compared to machines adapted to polish many wafers simultaneously. Adding more machines to make up the production rate difference per machine requires a higher capital investment in equipment and more factory floor space.
What is needed is a polishing machine having high throughput and a complex relative motion between a surface of a wafer to be polished and a polishing pad on a platen, for all parts of the surface of a large wafer.